Mehmet Özer, 030070050, 1st Week

1.Martensitic transformation (Group: material)

Previous answer:

A martensitic transformation is a structural phase transformation of the diffusionless and cooprative type, where the rearrangement of atoms occurs with relatively small displacements compared to inatomic distancesç There is a rigorous crystallographic connection between the lattices of the initial and final phases. The trasnsformation is of the first order. The martensitic transformation is called thermoelastic, when it is thermally reversible.

During martensitic transformation, the high-temperature phase,called austenit, transforms to the low- temperature phase, called martensite. As it is a firts-order structural phase transformation, the high-temperature asutenite and the low- temperature martensitic phases coexist in a specific temperature range. This is due to the elastic strains that accompany the nucleation and growth of the martensitic or austenitic phase. The austenite-martensite phase boundaries are fully or partially coherent. The elastic strains due to teh martensitic transformation increase with increasing martensite fraction. To compensate the transformation strains, different crystallographic domains are formed within the martensite. Macroscopically, they are often visible as paralel bands on the sample surface.

(J. Ping Liu, Nanoscale Magnetic Materials And Applications, p. 401)

New answer:

Martensitic transformation consists in the alteration of the distance between neighbouring atoms and it manifests itself as a change of crystallographic structure from face-centred-cubic γ parent phase to body-centred-cubic α′ product phase. The applied stress or the plastic strain influence the free energy change, which acts as the driving force and can cause the phase transformation even above the martensite start temperature Ms. The deformation-induced martensitic transformation can be related to the TRIP (transformation-induced plasticity) effect, resulting in the uniform, unrecoverable, macroscopic strain, which occurs in some high-strength metastable austenitic steels. Kinematically controlled plastic strain-induced martensitic transformation may be used as a method of creating functionally graded materials with “tailored” mechanical properties. The functionally graded materials belong to the family of modern engineering materials, that are characterised by gradually evolving micro-structure, composition, phase distribution, porosity, etc. They are designed to obtain optimal spatial variation of properties, adapted to the specific application. FGMs join advantages of composite and layered materials and eliminate such problems like material discontinuity as well as associated high stresses and initiation of cracks and damage at the boundaries between two constituents or two layers.

The FGMs can be easily obtained within the structural members made of metastable austenitic stainless steels by loading them above the yield point and inducing the γ − α′ phase transformation. It is possible to obtain various distributions of mechanical properties, generated by two-phase micro-structure of the material, depending on the distribution of plastic strain fields as a function of the shape of structure.

(M. Sitko, B. Skoczen, Effect of γ − α′ phase transformation on plastic adaptation to cyclic loads at cryogenic temperatures, International Journal of Solids and Structures (2012), p.613)

2.Life Cycle Costs (Group: Accounting)

Previous answer:

One of the externalities of DFM. Throughout their life cycles, certain products may incur some company or social costs which are not (or are rarely) accounted for the manufacturing cost. For example, products may contain toxic materials requiring special handling in disposal. Products may incur service and warranty costs. Although these costs may not appear in the manufacturing cost analysis, they should be considered before adopting a DFM decision.

(Kalpakjian S.,Manufacturing Engineering and Technology, 5th Edition, p.229)

New answer:

Life cycle cost is the total cost of ownership of machinery and equipment, including its cost of acquisition, operation, maintenance, conversion, and/or decommission (SAE 1999). LCC are summations of cost estimates from inception to disposal for both equipment and projects as determined by an analytical study and estimate of total costs experienced in annual time increments during the project life with consideration for the time value of money. The objective of LCC analysis is to choose the most cost effective approach from a series of alternatives (note alternatives is a plural word) to achieve the lowest long-term cost of ownership. LCC is an economic model over the project life span. Usually the cost of operation, maintenance, and disposal costs exceed all other first costs many times over (supporting costs are often 2-20 times greater than the initial procurement costs). The best balance among cost elements is achieved when the total LCC is minimized (Landers 1996). As with most engineering tools, LCC provides best results when both engineering art and science are merged with good judgment to build a sound business case for action.

(Barringer H. P., A Life Cycle Cost Summary, p.2)

3.CFD - Computational Fluid Dynamics (Group: analyze method)

Previous answer:

The physical aspects of any fluid flow are governed by the following three fundamental principles: 1)mass is conserved; 2)F=ma; and 3)energy is conserved. These fundamental principles can be expressed in terms of mathematical equations, which in their most general form are usually partial differential equations. CFD is, in part, the art of replacing the governing partial differential equations of fluid flow with numbers, and advancing these numbers in space and/or time to obtain a final numerical description of the complete flow field of interest. This is not an all-inclusive definition of CFD; there are some problems which allow the immediate solution of the flow field without advancing in time or space, and there are some applications which involve integral equations rather than partial differential equations. In any event, all such problems involve the manipulation of, and the solution for, numbers. The end product of CFD is indeed a collection of numbers, in contrast to a closed-form analytical solution.

 (Computational Fluid Dynamics, John F. Wendt, 3rd Edition, p6)

New answer:

The basic idea is to model the derivatives by finite differences. When this approach is used the entire flowfield must be discretized, with the field around the vehicle defined in terms of a mesh of grid points. We need to find the flowfield values at every mesh (or grid) point by writing down the discretized form of the governing equation at each mesh point. Discretizing the equations leads to a system of simultaneous algebraic equations. A large number of mesh points is usually required to accurately obtain the details of the flowfield, and this leads to a very large system of equations. Especially in three dimensions, this generates demanding requirements for computational resources. To obtain the solution over a complete three dimensional aerodynamic configuration millions of grid points are required!

Originally, CFD was only associated with the 2nd and 3rd items listed above. Then the problem with establishing a suitable mesh for arbitrary geometry became apparent, and the specialization of grid generation emerged. Finally, the availability of large computers and remote processing led to the need for work in the last two items cited. Not generally included in CFD per se, a current limiting factor in the further improvement in CFD capability is development of accurate turbulence models.

(W.H. Mason, Applied Computational Fluid Mechanics Volume 2, 8,1-2)

4.The Impact Test (Group:Testing method)

Previous answer:
When a material is subjected to a sudde, intense blow, in which the strain rate is extremely rapid, it may behave in a much more brittle manner than is observed in the tensile test. An iimpact test is often used to evaluate the brittleness of a material under these conditions. Many test procedures have been devised, including the Charpy test and the Izod test. Izod test is often used for nonmetalic materials. The test specimen may be either notched or unnotched; V-notched specimens better measure the resistance of the material to crack propagation. In the test, a heavy pendelum, starting as an elevation h0, swinging through its arc, strikes and breaks the specimen, and reaches a lower final elevation hf. If we know the initial and final elevations of the pendelum, we can calculate the difference in potencial energy. This difference is the impact energy absorbed by the specimen during failure. The ability of a material to withstand an impact blow is often referred to as the roughness of the material. The material properties obtained from a serşes of impact tests are transition temperature, notch sensitivity an relationship to the stress-strain diagram.
(Askeland D.R., The Science and Engineering of Materials, 3rd Ed., Pg. 149-150, Kayra Ermutlu)

New answer:

A material is regarded as being tough if it absorbs a large amount of energy in breaking. In a tension test, the energy per volume to cause failure is the area under the stress– strain curve and is the toughness in a tension test. However, the toughness under other forms of loading may be very different because toughness depends also on the degree to which deformation localizes. The total energy to cause failure depends on the deforming volume as well as on energy per volume. Charpy test: Impact tests are often used to assess the toughness of materials. The most common of these is the Charpy test. A notched bar is broken by a swinging pendulum. The energy absorbed in the fracture is measured by recording by how high the pendulum swings after the bar breaks. Figure 13.21 gives the details of the test geometry. The standard specimen has a cross section 10 mm by 10 mm. There is a 2-mm-deep V-notch with a radius of 0.25 mm. The pendulum’s mass and height are standardized. Sometimes bars with U or keyhole notches are employed instead. Occasionally subsized bars are tested.

One of the principal advantages of the Charpy test is that the toughness can easily be measured over a range of temperatures. A specimen can be heated or cooled to the specified temperature and then transferred to the Charpy machine and broken quicklye nough so that its temperature change is negligible. For many materials there is a narrow temperature range over which there is a large change of energy absorption and fracture appearance. It is common to define a transition temperature in this range. At temperatures belowthe transition temperature the fracture is brittle and absorbs little energy in a Charpy test. Above the transition temperature the fracture is ductile and absorbs a large amount of energy. Figure 13.22 shows typical results for steel.

(Hosford W.F., Mechanical Behaviour of Materials, pp.220,221)

5.Tooling Costs (Group: Accounting)

Previous answer:
These are the costs involved in making the tools, dies, molds, patterns, and special jigs and fixtures required for manufacturing a product. High tooling costs may be justified in high-volume production of a single item. The expected life of tools and dies and their obsolescences ( because of product changes ) also are important considerations.
(Kalpakjian S., Schmid S.R.,Manufacturing Engineering and Technology, 5th Edition, pg.1262)

New answer:

The manufacture of a component consumes resources (Figure 13.35), each of which has an associated cost. The final cost is the sum of expenses of all of the resources it consumes (detailed in Table 13.5). Thus the cost of producing a component of mass m entails the cost Cm ($/kg) of the materials and feedstocks from which it is made. It involves the cost of dedicated tooling Ct ($) and that of the capital equipment Cc ($) in which the tooling will be used. It requires time, chargeable at an overhead rate C_ oh (thus with units of $/hr), in which we include the cost of labor, administration, and general plant costs. It requires energy, which is sometimes charged against a process step if it is very energy intense but more commonly is treated as part of the overhead and lumped into C˙ oh, as we shall do here. Finally there is the cost of information, meaning research and development, royalty or license fees; this, too, we view as a cost per unit time and lump it into the overhead. Think now of the manufacture of a component (the “unit of output”) weighing m kg, made of a material costing Cm $/kg. The first contribution to the unit cost is that of the material mCm magnified by the factor 1/(1−f) where f is the scrap fraction—the fraction of the starting material that ends up as sprues, risers, turnings, rejects, or waste:

The cost Ct of a set of tooling—dies, molds, fixtures, and jigs—is what is called a dedicated cost: one that must be wholly assigned to the production run of this single component. It is written off against the numerical size n of the productionrun. Tooling wears out. If the run is a long one, replacement will be necessary. Thus tooling cost per unit takes the form

where nt is the number of units that a set of tooling can make before it has to be replaced, and Int is the integer function. The term in curly brackets simply increments the tooling cost by that of one tool set every time n exceeds nt.

(Ashby M., Material Selecting in Design and Manufacturing 4^th edition, pp.409-410)